It is too light (thickness of glass) to be a line insulator. Will it fit over a common light bulb ?

My first though is that it is the vessel part of a water dispenser for fowl like chickens. The missing base would have a ring/trough for the water to stand in and holes connecting the vessel to the trough, by which water could flow when the trough level got below the atmospheric pressure threshold and air burped up into the vessel, equalizing the water into the trough.

I doubt it has any electrical application. Most certainly not a pole top/line carrying insulator in the usual sense. The color is like nothing I have ever seen for a squirrel guard/transformer type application.

It looks like the top part of an old poultry waterer. It would sit full of water inverted on a dish like basin and as the birds drank water would flow out of it into the basin to re-fill it. That would explain the vent hole at the top too.

1) a "cupping " jar. It would have had a rubber bulb attached to the hole that would be squeezed and then the jar mouth would be smeared with Vaseline or some other sealant and then placed on the skin. Once affixed, the bulb would be released creating a vacuum in the jar and upon the surface of the skin. Handy for drawing out lanced boils or letting blood, and all sorts of "sports medicine " quackery as you will occasionally see athletes that have had this done for some odd reason, leaving their skin spotted with round bruises where the jars were applied. (see Mike Phelps, Rio Olympics )

2) a breast pump. Same basic procedure as above, but applied to a more interesting region of the female body...